How to prepare: Preheat the oven to 180°C. Place the butternut on a baking tray, drizzle with half the oil and sprinkle with salt and pepper. Roast for 15 to 20 minutes, until tender. Remove and set aside. Then turn the oven on to grill. Meanwhile, heat the remaining oil in an ovenproof frying pan, fry the onion until soft. Add the spinach and cook for 1 minute, until just wilted. Remove pan from the heat and arrange the butternut and feta cheese on the spinach. Whisk together the eggs, milk, grated cheese and seasoning. Pour over the butternut, spinach mixture. Return pan to a medium heat and cook for 5 to 10 minutes, until the bottom of the frittata is cooked. Then place pan under the grill and cook the top for a few minutes, until golden. Remove and slice into wedges. The frittata can be served hot or cold.